1. Field of the Invention
The present invention relates to a controller for controlling the control amount of a plant that has nonlinearities, such as hysteresis and dead zone.
2. Description of the Related Art
A controller of the above-mentioned kind has already been proposed by the present assignee as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-63003. This controller controls the cam phase as a control amount by inputting a control input to an electromagnetically-driven variable cam phase mechanism as a plant, and includes a two-degree-of-freedom sliding mode controller and a DSM controller. The two-degree-of-freedom sliding mode controller calculates an SLD control input for causing the cam phase to converge to a target value, with a target value filter-type two-degree-of-freedom sliding mode control algorithm.
Further, the DSM controller calculates the control input to the variable cam phase mechanism, by modulating the calculated SLD control input with an algorithm to which is applied a ΔΣ modulation algorithm, such that the control input is frequently inverted with a predetermined amplitude with respect to a predetermined value as the center. As a result, even when controlling the cam phase via the variable cam phase mechanism having nonlinearities, it is possible to control the cam phase by the control input frequently repeating the invention such that the cam phase is caused to converge to the target value while compensating for the nonlinearities. This makes it possible to control the cam phase in a fine-grained manner, and maintain both the resolution and the control accuracy of cam phase control at high levels.
Assuming that the controller as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-63003 is applied to a plant including a control region in which nonlinearities are temporarily very strongly exhibited and a control region in which the nonlinearities are hardly exhibited, when the plant is in the control region in which the nonlinearities are exhibited, there occurs a state in which the control amount largely deviates from the target value, and the SLD control input is calculated as a value that changes over a larger variation range than when the plant is in the control region in which the nonlinearities are hardly exhibited. For this reason, to compensate for the nonlinearities, the amplitude of the control input is required to be set to a value large enough to cover the SLD control input having such a larger variation range. If the amplitude of the control input is set to such a value, however, when the plant is in the control region where the nonlinearities are hardly exhibited, under conditions of the sensitivity of the control amount to the control input being low, particularly under conditions of the frequency sensitivity, more specifically, the high-frequency cut properties being low, the large amplitude of the control input is reflected on the control amount in a noise-like fashion, which can degrade the resolution and accuracy of the control.